This invention pertains to shape memory alloys. In a more specific aspect the invention relates to rotary actuators incorporating shape memory alloys.
As pointed out in U.S. Pat. No. 5,127,228 shape memory alloys are a group of metals which have mechanical properties having to do with shape memory. These alloys, when deformed at temperatures below their martensite temperatures, resume the shapes which they had prior to the deformation if they are heated to temperatures above their austensite temperatures. Such alloys include Cu-Zn-Al alloys, Cu-Al-Ni alloys, Ni-Ti alloys and the like.
One application of shape memory alloys has been their use in actuators. Actuators include members which, when deformed in some manner, return to their original states. One type of actuator is a bias spring type. When heated, a deformed shape metal alloy (SMA) returns to its original state, overcoming the spring bias. When the SMA member is allowed to cool the spring again deforms the SMA member.
Another type of actuator, termed a differential actuator, uses two SMA members mechanically connected in series. Heating one performs work in one direction, thus deforming the other one. When the other SMA is heated the action is reversed. Such bidirectional devices can be found in U.S. Pat. No. 4,010,455, and U.S. Pat. No. 4,887,430. In U.S. Pat. No. 5,127,228 concentric members are described. One member is in the form of a sleeve surrounding a cylinder, both being constructed of shape memory alloys.
It has been pointed out that bias spring type actuators do not have two positions which are stable. The differential types have several gears such as those in U.S. Pat. No. 5,127,228, or other moving parts rendering them bulky, complex and costly. Hence in the field of rotary actuators there is room for improvement. Such an improved rotary actuator is provided by this invention.